1. Field of the Invention
This invention relates to zoom lenses composed of two lens units in simple form suited to lens-shutter cameras, video cameras, etc. and, more particularly, to zoom lenses of shortened total length (distance from the first lens surface to the image plane) in simple form with the number of lens members in each of the lens units being extremely reduced and the aberrations being well corrected.
2. Description of the Related Art
Along the progress of minimization of the size of the lens-shutter camera or video camera have come an increasing number of proposals for zoom lenses of short total length and simple form.
In, for example, Japanese Laid-Open Patent Applications Nos. Sho 57-201213, Sho 60-170816, Sho 60-191216 and Sho 62-56917, the zoom lens is constructed with a first lens unit, when counted from the object side, of positive refractive power and a second lens unit of negative refractive power, totaling two lens units, both lens units moving in differential relation to effect variation of the magnification. Thus, the size is reduced.
Using such a power arrangement, the prior art in these documents has realized a zoom lens of relatively shortened back focal distance, while still maintaining high optical performance to be achieved with the limitation of the total length to a minimum.
Besides this, a zoom lens comprising a first lens unit of positive refractive power and a second lens unit of negative refractive power with the separation between both lens unit being varied to vary the magnification is proposed in, for example, Japanese Laid-Open Patent Application No. Sho 62-251710. In the same document, the first lens unit is constructed from three lenses or in the form of the triplet type, and the second lens unit from two lenses, or a positive lens and a negative lens. The five lenses as a whole constitute a zoom lens having a zoom ratio of about 1.5 and a simple form.
Also, in Japanese Laid-Open Patent Application No. Sho 63-276013, the first lens unit is constructed from two lenses, or a negative lens and a positive lens, and the second lens unit from two lenses, or a positive lens and a negative lens, totaling four lenses. Of these, a plurality of lenses are gradient-index lenses. With these, a zoom lens having a zoom ratio of about 2 and a simple form is produced.
In general, in the above-described type of zoom lens, or the type having only two lens units, to afford a simplification of the lens form, it is recommended that each lens group is constructed with as few a number of lenses as possible.
However, to allow the prescribed zoom ratio and photographic image angle to be obtained within this framework, proper rules of design for the individual lens units must be set forth. Otherwise, the range of variation of all aberrations with zooming would increase. So, these would become difficult to correct well.
Also, the proposal of the aforesaid Japanese Laid-Open Patent Application No. Sho 63-276013, or the method of simplifying the lens form by using the gradient-index lenses, is associated with a difficult problem of manufacturing gradient-index lenses in tight tolerances.